Processes for the co-production of acetic acid and dimethyl ether may be carried out by the catalytic dehydration and hydrolysis of mixtures of methanol and methyl acetate. Such co-production processes are known from, for example WO 2011/027105, WO 2013/124404 and WO 2013/124423.
WO 2011/027105 describes a process for the production of acetic acid and dimethyl ether by contacting methanol and methyl acetate with a catalyst composition at a temperature in the range 140 to 250° C. wherein the catalyst composition contains a zeolite having a 2-dimensional channel system comprising at least one channel which has a 10-membered ring.
WO 2013/124404 describes a process for the co-production of acetic acid and dimethyl ether from a mixture of methanol and methyl acetate by contacting the mixture at a temperature from 200 to 260° C. with a catalyst composition comprising a zeolite possessing a 2-dimensional channel system comprising at least one channel having a 10-membered ring and a silica:alumina molar ratio of at least 22.
WO 2013/124423 describes a process for the production of acetic acid and dimethyl ether by contacting a mixture of methanol and methyl acetate with a zeolite catalyst wherein the zeolite has a 2-dimensional channel system comprising at least one channel having a 10-membered ring and having at least 5% of its cation exchange capacity occupied by one or more alkali metal cations.
Although water is produced in such co-production processes via the dehydration of methanol, a water feed can also be supplied to the process.
In general, methanol is synthesised by converting gaseous mixtures of carbon monoxide, hydrogen and optionally carbon dioxide in the presence of a catalyst, typically a catalyst containing copper as the catalytically active component to produce a crude methanol product. As a result of side-reactions low levels of methyl formate may be produced in the methanol synthesis process.
In processes for the co-production of acetic acid and dimethyl ether, the presence of methyl formate in one or both of the methanol and methyl acetate feedstocks is undesirable as it results in the generation of formic acid which is difficult to separate from acetic acid by conventional fractional distillation techniques owing to the closeness of their boiling points. Instead more complex extractive distillation methods have been employed to achieve acetic acid product purities. Complex methods of this type for the separation of formic acid from acetic acid are described in, for example U.S. Pat. No. 4,692,219 and U.S. Pat. No. 5,227,029.